The present invention relates, in general, to fluid pressure responsive brake systems and, more particularly the invention relates to a pneumatic or hydraulic brake for the trailer of a vehicle.
One particular type of such a brake system is described by German Patent DE-OS No. 32 07 793. This prior art type of brake system is controlled both pneumatically and electrically.
The pneumatic control is exercised by the brake circuit of the motor vehicle via a connectable trailer brake line, which is connected with the control input of a trailer brake valve. Connected in parallel to the pneumatic brake force regulation of the trailer brake, the electrical control is exercised by means of a pressure sensor, which measures the brake control pressure in the trailer brake line. The brake pressure, converted by the pressure sensor into an electrical signal, is conducted to electrically regulated pressure control valves which correspond to the respective brake cylinders.
It is known that this electrical signal can be modified by means of an electrical control unit connected in the trailer brake system by electrical signals from axle load and/or wheel speed sensors, for example. In addition, an electrical control signal can be transmitted to the trailer brake circuit via an additional electrical line if desired; such a control signal could be produced, for example, by the tractor brake pedal.
In spite of the above-mentioned types of electrical controls of the trailer brake circuit, an essential feature in this installation is the capability of a redundant pneumatic transmission of the brake pressure, because if there is an interruption in the electrical feed, e.g., if the trailer separates from the tractor, or while the tractor is parked with the trailer connected, the electrical system would generally become ineffective. The braking of the trailer in these cases is necessary and is exercised via the pneumatic brake circuit, in which, when the trailer brake line and/or trailer reservoir is vented, there can be a pressurization of the brake cylinder, i.e., a setting of the trailer brake, by the trailer reservoir via the reversed trailer brake valve.
A setting of the brakes with a trailer brake circuit not receiving electrical power, so that the pressure control valves would place the reservoir circuit in communication with the feed cylinders, would result during travel in an undesirable sudden braking of the trailer. A braking action reduced for this special case, e.g., by the use of a throttle between the pressure control valves and the brake cylinder, would be impermissible, because then the service braking action would also be throttled.